1. Field of the Invention
The present invention relates generally to wet cleaning of substrates during semiconductor wafer fabrication, and more particularly, to techniques, systems and apparatus for evaluating the effectiveness of techniques used to dry substrates following a wet clean procedure.
2. Description of the Related Art
In the fabrication of semiconductor devices, there is a need to perform wet cleaning of substrates at various stages of the fabrication process. Typically, integrated circuit devices are in the form of multi-level structures. At the substrate level, transistor devices having diffusion regions are formed over and into silicon substrates. In subsequent levels, interconnect metallization lines are patterned and electrically connected to the transistor devices to define the desired functional device. As is well known, patterned conductive layers are insulated from other conductive layers by dielectric materials, such as silicon dioxide. At each metallization level there is a need to planarize metal or associated dielectric material. Without planarization, fabrication of additional metallization layers becomes substantially more difficult due to the higher variations in the surface topography. In some applications, metallization line patterns are formed in the dielectric material, and then metal CMP operations are performed to remove excess metallization.
Following each CMP operation, a wet clean of the substrate is performed. The wet clean is designed to wash away any by-products of the fabrication process, remove contaminants, and to achieve and maintain the necessary degree of cleanliness essential to proceed to a subsequent fabrication operation. As transistor device structures become smaller and more complex, the precision required to achieve and maintain structure definition demands exacting standards of cleanliness be maintained in all process operations. If a wet clean is incomplete or ineffective, or if a post-wet clean drying is incomplete or ineffective, then unacceptable residue or contaminants are introduced into the processing environment.
Rinsing and drying techniques, methods, and apparatus are plentiful and known in the art, and incorporate such operations as rinsing and scrubbing, immersion, and the application of thermal, mechanical, chemical, electrical, or sonic energy and the like to remove or displace water and dry the substrate. While some scrub and rinse operations may employ acids or bases for vigorous interaction with fabrication by-products, deionized water (DIW) is commonly used to perform a final rinse before the desired drying technique is performed.
One common drying technique is known as spin, rinse and dry (SRD). SRD uses mechanical, centrifugal, energy to rid the substrate of water by spinning the substrate until dry. FIG. 1 shows a typical prior art SRD process and apparatus 10. An SRD apparatus 10 typically includes a substrate mounting plate 18 within a bowl 12 and mounted on a shaft 20 that is configured to rotate and thus spin the substrate 14. The substrate 14 is attached to the substrate mounting plate 18 with mounting pins 16 configured to maintain the substrate 14 in a horizontal orientation, firmly affixed to the substrate mounting plate 18 so that rapid rotation of the substrate mounting plate 18 spins the substrate 14 and forces the water from the substrate 18. DIW 26 is typically dispensed from a nozzle 24 which is positioned over the substrate 14 and connected to a DIW supply 22.
The SRD process essentially includes applying DIW or rinsing 28, and spinning the substrate dry 30. In some configurations, the substrate 14 is rinsed 28 while spinning to ensure thorough rinsing 28, and then spun to dry 30. The spinning of the substrate 14 uses centrifugal energy to force water from the substrate 14 surface, and can be enhanced with the introduction of an inert gas such as Nitrogen or an inert gas vapor to displace any water that is not completely removed by spinning. Additional variations include heating the DIW, heating the SRD environment, heating the inert gas, and the like.
Another common drying technique is known as a Marangoni technique. Marangoni drying (not shown) typically includes using a chemical drying fluid or solvent such as isopropyl alcohol (IPA) to introduce favorable surface tension gradients facilitating removal of water from the surface of a substrate. Variations of the Marangoni technique also include the introduction of an inert gas such as Nitrogen as a carrier gas for IPA vapor delivery.
Additionally, another known drying technique involves the replacement of DIW with another volatile compound.
Whichever method or combination of methods is employed to dry a substrate, effective drying is essential to continued fabrication. As is known, contaminates can damage or destroy features that are formed in single dies, groups of dies, or entire wafers.
Any water remaining on the substrate after the drying process evaporates. Water allowed to evaporate introduces contaminants as evidenced by the water marks or stains caused by residual solids from evaporated water. It is therefore desirable to evaluate drying techniques used, recognizing that the techniques are more or less effective depending on such factors as the type of substrate being processed, fabrication materials, processing environment, and the like. Common methods of evaluating the effectiveness of selected drying techniques include visual inspection, electrical analysis and mass analysis.
Visual inspection of substrates is generally effective for blanket film substrates as the surface of the substrate is smooth and easily inspected for remaining water marks. Patterned substrates, however, are difficult to inspect visually as water can be trapped in patterned features and not visible. Visual inspection is therefore ineffective for drying technique evaluation of patterned substrates.
Electrical analysis can be effective for specially prepared test structures after subjecting such structures to an electrical test such as TVS and the like. Such electrical analysis, however, is costly.
Mass analysis is a comparative evaluation of wet and dry substrates. Typically, mass analysis includes an initial drying operation followed by weighing the substrate and then, after some time, re-weighing the substrate to determine if a change in mass has or has not occurred. Although mass analysis is not subject to the same limitations presented by visual inspection and electrical analysis in the evaluation of patterned substrates, mass analysis is cumbersome, time consuming, and far less accurate than other methods.
What is needed is a method to evaluate advanced drying techniques used in the fabrication of semiconductor substrates. The method should include a way to accurately and precisely analyze a substrate that has been dried for any trace amount of residual contamination, and to use the results of the analysis to select, modify, or adjust the drying technique to ensure complete substrate drying in a contaminate-free environment.
Broadly speaking, the present invention fills these needs by providing a system and apparatus for evaluating drying techniques. The system and apparatus includes applying a compound to a final rinse after a wet clean of a substrate, drying the wafer in accordance with the selected drying technique, and then using a confocal microscope configured to analyze any residual compound on the substrate after the drying method is completed. The present invention can be implemented in numerous ways, including as a process, an apparatus, a system, a device, or a method. Exemplary embodiments of the present invention are described below.
In accordance with one aspect of the invention, an apparatus for measuring the effectiveness of a wafer drying operation is provided. The apparatus includes a laser and a focusing lens to direct laser energy onto a patterned surface of a wafer. A dichroic mirror is provided to direct fluorescent energy emanating from the surface of the wafer. The apparatus further includes a first photomultiplier for capturing an image of the detected fluorescent energy, a partial reflector for directing reflected laser energy from the surface of the wafer, and a second photomultiplier for capturing an image of the reflected laser energy received from the partial reflector. The apparatus then uses the image of the detected fluorescent energy and the reflected laser energy to produce a composite image for evaluating the effectiveness of the wafer drying operation.
In accordance with another aspect of the invention, a system for determining wafer drying effectiveness is provided. The system includes an argon laser to apply laser energy, and a focusing lens to direct the laser energy onto a patterned surface of a wafer. The system further provides a confocal aperture that receives the laser energy from the focusing lens, and a microscope objective that directs the laser energy onto the patterned surface of the wafer. The system next provides a dichroic mirror that directs fluorescent energy received from the surface of the wafer to a first photomultiplier. A partial reflector directs reflected laser energy from the surface of the wafer to a second photomultiplier, and the fluorescent energy and the laser energy are used to evaluate the effectiveness of the wafer drying operation.
In accordance with yet another aspect, an apparatus for quantifying the effectiveness of a wafer drying process is provided. The apparatus includes a laser configured to apply laser energy at a wavelength of about 488 nm, and a focusing lens that directs the laser energy onto a patterned surface of a wafer. The apparatus further provides a dichroic mirror that directs fluorescent energy of a wavelength between about 550 nm and about 650 nm emanating from the surface of the wafer to a first photomultiplier that captures an image of the detected fluorescent energy. The apparatus further includes a partial reflector that directs reflected laser energy of a wavelength of about 488 nm from the surface of the wafer to a second photomultiplier that captures an image of the reflected laser energy received from the partial reflector. The apparatus the uses the image of the detected fluorescent energy and the reflected laser energy to produce a composite image which is used to evaluate the effectiveness of the wafer drying operation.
In yet another embodiment, an apparatus is provided. The apparatus includes an argon laser to apply laser energy and a focusing lens that directs the laser energy onto a patterned surface of a wafer. The apparatus further provides a dichroic mirror to direct fluorescent energy emanating from the surface of the wafer at a first photomultiplier. The first photomultiplier captures an image of the detected fluorescent energy. A partial reflector directs reflected laser energy from the surface of the wafer at a second photomultiplier which captures an image of the reflected laser energy received from the partial reflector. The apparatus then uses the image of the detected fluorescent energy and the reflected laser energy to produce a composite image to evaluate the effectiveness of the wafer drying operation.
In still a further embodiment, an apparatus is provided. The apparatus includes an argon laser that applies laser energy at a wavelength of about 488 nm, and a focusing lens that directs the laser energy onto a patterned surface of a wafer. The apparatus further provides a dichroic mirror that directs fluorescent energy of wavelengths between about 550 nm and about 650 nm emanating from the surface of the wafer at a first photomultiplier which captures an image of the detected fluorescent energy. A partial reflector directs reflected laser energy of wavelengths of about 488 nm from the surface of the wafer at a second photomultiplier which captures an image of the reflected laser energy received from the partial reflector. The image of the detected fluorescent energy and the reflected laser energy is used to produce a composite image for evaluating the effectiveness of the wafer drying operation.
The advantages of the present invention are numerous. One notable benefit and advantage of the invention is the apparatus provide non-biased, quantitative comparison of different drying techniques on patterned wafers. The most commonly utilized prior art of wafer inspection following wet cleans provides no quantitative evaluation, and suffers significant shortcomings as previously detailed. The present invention can be implemented for a plurality of drying techniques, and provides usable, measurable data to evaluate the effectiveness of the selected technique for specific structures, geometries, complexities, and the like.
Another benefit is the cost effectiveness of the present invention. The apparatus are not complicated, and do not require implementation with production, device wafers, but can be used in the RandD stage of production, and with test pattern wafers. This further allows effective evaluation of drying technologies at the stage of concept and feasibility studies, and thus reduces the associated cost of new cleaning tool development.
An additional benefit is that the present invention is an efficient and simple apparatus. Implementation is easily and efficiently incorporated into existing infrastructure, and vastly increases the efficiency of development and production.
Other advantages of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.